Three general types of transducers are presently available for measuring pressure: differential, sealed, and absolute. The differential transducer (also called a gauge transducer) measures the difference between pressures applied to what are known as the "front" and "back" sides of its sensor. The sensor consists of a resistor network such as a Wheatstone bridge disposed on a diaphragm such that when pressure is applied to either side of the sensor the diaphragm is deformed, thus changing the resistance of the network. When different pressures are applied to either side of the diaphragm, the change in resistance in the network can be correlated to the difference in pressures applied to the front and back of the sensor.
The sealed transducer works on the same principle as the differential transducer except that the back of the transducer is sealed such that there is a constant pressure applied to the back of the diaphragm. Thus the sealed transducer will produce signals indicative of the difference between whatever pressure is applied to the front of the sensor and the constant pressure applied to the back.
The absolute transducer is a special type of sealed transducer wherein the constant pressure applied to the back of the transducer is zero. In other words, the back of the transducer is sealed with a hard vacuum. Thus, the absolute transducer will produce signals indicative of whatever pressure is applied to the front of the sensor relative to the vacuum.
The most commonly employed of these transducers in the measurement of pressure in a well is the differential transducer. For a number of years, differential transducers have been used to measure the level of a liquid in an unsealed well. This is a relatively simple measurement which entails lowering the differential transducer to a point below the surface of the liquid in the well, venting the back side of the transducer to atmospheric pressure outside of the well, and connecting the resistor network disposed on the diaphragm of the transducer to a data analyzer. The front side of the transducer then measures the pressure due to the column of liquid above the transducer plus the atmospheric pressure which pushes down on the surface of the liquid. By venting the back side of the transducer to conditions outside of the well, the effect of atmospheric pressure is negated and the transducer sends signals to the data analyzer indicative of the pressure caused by the column of liquid above the differential transducer. Since the depth to which the transducer was lowered is known, the data analyzer can then determine the level of liquid in the well.
If the same test were performed with an absolute or sealed transducer, the effect of atmospheric pressure on the surface of the liquid would not be taken into account, and an inaccurate level reading would result. It would also be inaccurate to merely program the data analyzer to subtract what is traditionally thought of as atmospheric pressure from the pressure read by an absolute transducer. This would not account for barometric changes in the atmosphere which could significantly affect results in a long-term test. Thus, differential transducers have traditionally been employed when measuring the level of liquid in an unsealed well. In fact, differential transducers are the most commonly used type of transducer in a variety of procedures because of their ability to compensate for changes in atmospheric pressure.
Recently, however, procedures have been developed which require a well to be sealed air-tight and an artificial pressure to be created in the well. When differential transducers have been used in these new procedures to measure pressure inside of the well, inaccurate readings have been produced. Since the pressure on the surface of the liquid is no longer the same as the pressure in the atmosphere, when the back side of the differential transducer is vented to the atmosphere, the pressure on the surface of the liquid is no longer canceled out. Thus, the signals produced by the transducer are no longer indicative of only the pressure caused by the column of water above the transducer.
One example of a procedure in which it is necessary to create an artificial pressure within a well is "soil vapor extraction." Soil vapor extraction refers to a method of removing contaminants from the soil by lowering the pressure in a well which is situated in or near the contaminated soil. If contaminants, such as gasoline for example, become trapped in soil a certain depth beneath the ground and above the table of groundwater, the contaminants can be removed through a well dug through the area of contamination to the groundwater table. When the well is sealed and a vacuum applied to it in order to lower the pressure, contaminants in the soil will volatilize and move into the well in vapor form. Once in the well, the vapor contaminants are removed by the vacuum and disposed of properly. However, when the vacuum is applied to the well, the water from the groundwater table tends to rise, and if the water rises to the level of the contaminants, the contaminants will not volatilize and will remain trapped in the soil. The well is therefore provided with a pump which can be used to remove enough water to keep the water level in the well below the level of the contaminants. In order to use the pump effectively, the level of the water in the well must be known at any given time.
When a differential transducer with its back side vented to the atmosphere has been used to measure the water level, the readings have been found to be in error for the reasons outlined above. Therefore, an incorrect water level is maintained. Absolute transducers have been used to obviate these errors, however, two absolute transducers are then required for each well. If a large number of wells are being measured at any one time, the use of two absolute transducers per well could require a significant investment. In addition, since most users already own differential transducers which can be used in other procedures, the use of absolute transducers is even less cost-effective.
Another procedure for which a well is sealed and an artificial pressure applied is a "pneumatic slug test." A slug test measures how fast water will move through an aquifer by either increasing or decreasing the amount of water in the well and timing how long it takes for the well to return to its original level. For years this test was performed using an unsealed well by either pumping water out of the well or adding water to the well and waiting for the well to recover to its original level. There are problems associated with each of these methods. Namely, if water is removed and the well is contaminated, then how to dispose of the contaminated water can be a problem. Alternatively, if water is added and the well is pristine, then there is a risk that the added water will contaminate the well.
Therefore, recently, technology has been developed to perform the slug test pneumatically, without adding or removing any water. In a pneumatic slug test the well is sealed, and the pressure in the well is then raised or lowered. This causes the water level in the well to drop or rise accordingly. The artificial pressure is then removed, and the time it takes for the water in the well to recover to its original level is measured. This avoids the potential problems of disposal and contamination. However, in order to perform this test, it is necessary to be able to accurately measure the level of the water in the well. It has been discovered that when using a differential transducer with its back side vented to the atmosphere to measure the water level when the artificial pressure is applied, inaccuracies occur for the reasons outlined above.
Another test which requires that a well be sealed and an artificial pressure applied is known as a "soil permeability test." This test measures the permeability of soil by testing how well air can pass through it. A soil permeability test is usually performed prior to a soil vapor extraction procedure in order to determine if soil vapor extraction is a viable method for cleaning up a contaminated plot of soil. Obviously, soil which is mostly made up of clay will not be as permeable as more granular soil. If the soil is not permeable enough, the contaminants in the soil are less likely to volatilize and another method of soil cleansing may be more effective.
A soil permeability test is performed using a plurality of wells. First, an extraction well is sealed and a pressure is applied to it. Then one or more monitoring wells near the extraction well are created and sealed. The pressure is measured in each monitoring well to gauge the effect of the pressure applied to the extraction well. In order to correlate the pressure changes in the monitoring wells to the permeability of soil between the wells, it is necessary to measure the true pressure being applied to the extraction well. The use of a differential transducer to measure the pressure applied to the extraction well is hampered in that if a vacuum is applied to an extraction well in which a differential transducer is suspended above the water level, the differential transducer will malfunction, because the diaphragm will be deformed in the wrong direction. The data analyzer will read an open circuit in the resistor network and no measurement will be possible. Absolute transducers which are designed to measure negative pressures (pressures below atmospheric) could be used, however, as discussed above, the use of absolute transducers is not cost effective given that they cannot be employed in all tests.